Kauffman (1977) has presented a model of Cretaceous paleogeography and paleoclimatology as the supercontinent of Pangea broke up and the Atlantic Ocean opened. A circum-equatorial ocean, the Tethys Seaway, dominated world paleogeography, stretching across the America, northern Africa, and southern Europe into the Paleo-Indian Ocean (where India was still a drifting subcontinent). Branching off this east-west ocean were several north-south epicontinental seaways including the Western Interior seaway and the Northern Atlantic Seaway, both stretching from the Tethys northward into the paleo-Arctic Ocean. The Western Interior Seaway was characteristically a shallow, epicontinental seaway, perhaps little more than a hundred meters deep while the northern Atlantic was a true oceanic area floored by oceanic crust extruded along the mid-Atlantic Ridge as the ocean basin opened by sea floor spreading. The Atlantic Ocean was certainly a deeper, more oceanic body of water less prone to the vagaries of local climate and tectonic activity. Within this setting the primitive decapods underwent several major adaptive radiations and formed the lineages leading to modern decapod faunas.
Western Interior Seaway
The Western Interior Seaway as described by Kauffman (1977) is a complex epicontinental sea extending about 4,800 km from Arctic Canada to the Tethys Seaway and approximately 1,600 km wide from a mobile belt along its western edge in Nevada and Utah to a stable shallow shelf along its east side. Within this basin water depths were variable and are inferred by Kauffman (1977) to have been on the order of 100m, a value accepted by most Cretaceous workers. Other authors cite much deeper conditions (Eicher 1969). The active western margin was the source of coarse clastics deposited in rapidly prograding deltaic complexes into rapidly subsiding basins resulting in a complex series of clastic wedges. Farther to the east the Western Interior Seaway was a stable platform which received the suspended fine clastics from the western part of the basin and surrounding low-lying hills of eastern North America deposited as thick units of mudstone and resulting in classic interfingering patterns with coarser clastics to the west as tectonism pulsed throughout the Cretaceous. Paleocurrent patterns for the Tethys Seaway and the Western Interior Seaway were presented by Kauffman (1977; fig. 10) in assessing distribution of larvae throughout the basin.
The interfingering Cretaceous lithosomes also received air-transported volcanic ash from volcanos in the tectonically active Rocky Mountains to the west, resulting in deposition of numerous layers of bentonite on the Cretaceous sea bottom. These isochronous, pyroclastic events must have dramatically altered bottom conditions, water chemistry, and atmospheric conditions, leading to dramatic changes in Cretaceous ecosystems and biotas. As Laramide tectonism progressed, the Western Interior Basin was inundated by sediment from the west causing progradation of large deltas and restriction of the basin in Maastrichtian time to a small Paleocene remnant sea centered in North Dakota, the Cannonball Sea.
The Cretaceous sediments of the Western Interior carry extensive and beautifully preserved faunas, primarily encased in concretionary limestone nodules. These faunas include rapidly evolving cephalopod molluscs such as Baculites, Didymoceras, Hoploscaphites, and many other taxa which have been developed into a well-documented chronology based on faunal succession, bentonite stratigraphy, and radiometric dating. This chronology allows other physical and biologic events, such as periodic appearances of Decapod-Lagerstätten to be precisely dated.
The Mississippi Embayment is a Late Cretaceous and Tertiary seaway that extended from the Tethys Seaway northward into southern Illinois as a well defined depositional trough characterized by shallow shelf conditions and relative tectonic stability (Russell, et al. 1982). Resultant sediments consist of shelf chalks and muddy chalks and nearshore clastics consisting of basal gravels, sands, and muddy sands. Within these sediments (Stephenson and Monroe 1940) are contained well known faunas of the Selma Chalk, Ripley Formation, and the Coon Creek Formations, including several extensive decapod faunas from Blue Springs, Mississippi; Coon Creek, Tennessee; and Braggs, Alabama.
Northern Atlantic Coastal Plain
The history of geological study of the Cretaceous deposits of the Northern Atlantic Coastal Plain was reviewed by Owens, et al. (1970). Geologic studies of these rocks has been hampered by low relief, dense vegetative cover, thick soils, repeated facies, poor preservation of invertebrates, and many other factors. Data and diagrams presented by Owens et al. indicate the Late Cretaceous sediments thin southward and some units pinch out before reaching the Delmarva Peninsula leaving the Merchantville, Englishtown, Marshalltown, and Mount Laurel Sand (oldest to youngest) representing a series of clastic and calcareous clastic lithosomes deposited in nearshore environments along the wester margin of the opening Atlantic Ocean. These sediments along the Delmarva Peninsula have yielded a diverse and abundant decapod fauna collected by Harry Mendryk from dredging soil exhumed from the Chesapeake & Delaware Canal during the 1970‘s, mostly assignable to the early Campanian Merchantville Formation.
Western Gulf Coastal Plain and Mexico
The depositional history of the Western Gulf Coastal Plain and Mexico during the Late Cretaceous was very similar to that for the Western Interior Seaway. Northeastern Mexico was dominated by prograding deltaic systems through Campanian and Maastrichtian time. Deposition of sediments and local subsidence of basins produced thicknesses up to 1000 m. Dakoticancrid crabs from Mexico have been reported for the Maastrichtian sediments of the Difunta Group (Vega and Feldmann, 1991), and Cardenas Formation (Vega, Feldmann, and Sour-Tovar, 1995).
The Difunta Group is an Upper Cretaceous-Eocene sequence of terrigenous rocks, located in the states of Coahuila and Nuevo Leon. Two deltaic systems prograded from the west and northwest, filling the Parras and La Popa basins (McBride et al., 1974). Although Maastrichtian dakoticancrid crabs seem to be more abundant in the shallow calcarenites and sandstones of the La Popa basin, some specimens have been collected recently from the Parras basin. Deposited north of the structural front of the Sierra Madre Oriental, the Difunta Group was affected by a system of NW-SE faults. Diapirs in the La Popa basin were the result of plastic deformation and pressure over the underlying Minas Viejas anhydrites. It is probable that this feature resulted in local subsidence and basin filling episodes, which prevailed until early Eocene times (Vega et al., 1989). Uplift of the Sierra Madre Oriental took place through the Late Cretaceous; thus, depositional basins of the Difunta Group were bordered by this feature to the south and southeast, by the Coahuila Platform to the north and northwest, and by the Tamaulipas arch to the east and northeast (McBride et al., 1974).
The southernmost occurrence of dakoticancrid crabs has been recorded from the sandstones and shales of the Maastrichtian Cardenas Formation, on the eastern margin and central portion of the Sierra Madre Oriental, in San Luis Potosi state (Vega, Feldmann, and Sour-Tovar, 1995). The Cardenas Formation comprises reefal facies in the southern part of its exposure area (Myers, 1968), while to the north it becomes more terrigenous, with an almost uniform lithology of fine sandstones and shales. Terrigenous material was supplied from the west, from the rising occidental margin of Mexico. Uplift of the Sierra Madre Oriental during Late Cretaceous formed a barrier to the east, thus deposition of Cardenas sands and muds took place in an elongated, nearly north-south corridor-shaped lagoon. Farther east, pelagic sediments of the Mendes Formation were deposited beyond this barrier, onto the Gulf Coastal Plain.
The Difunta Group and Cardenas Formation have closer faunal affinities with rocks of the Mississippi Embayment than with correlative units of southwest Texas. The Campanian-Maastrichtian Aguja Formation comprises an eastward-thinning series of paralic and marine sandstones interbedded with shale and lignite deposited in a coastal plain environment. Two progradational cycles are represented in these deposits (Lehman, 1986). As a difference of the preservational style with Mexican specimens, decaapods from the Big Bend area are phosphatized exuviae of species more closely related to the Western Interior decapod fauna.
Biogeographic range of Dakoticancrids
Three major biogeographic patterns of occurrence of the dakoticancrids are discernable; each dominated by a genus and having relatively discrete boundaries and each with a few known outside occurrences. The occurrences of Tetracarcinus subquadratus are largely confined to the Northern Atlantic Coastal Plain and the Mississippi Embayment with one specimen also known from Wyoming. Avitelmessus grapsoideus is found dominantly in the Mississippi Embayment and Eastern Gulf Coastal Plain but is also known from North Carolina. Dakoticancer overanus is endemic to the Western Interior Seaway and Dakoticancer australis is abundant in the Mississippi Embayment but is also present in northeastern Mexico. These ranges might be extended by additional collecting, but the coherence of their distributions has remained relatively constant for a significant time, leading us to believe the distributions are real biogeographic patterns, not artifacts of collecting biases.